Data communication for exposure control, automatic focus control, etc. is performed between a camera serving as an image pickup apparatus and an interchangeable lens detachably attached to the camera. The amount of data to be communicated is across multiple transmission units in order to perform the control with a high precision matched with the optical characteristics of each interchangeable lens, and the information is required to be sequentially updated each time the zooming position of the interchangeable lens or the position of a range ring is varied. Accordingly, it is necessary for the camera to frequently perform such communication.
Synchronous serial communication using clock signals has been adopted as a communication method between a camera and an interchangeable lens. FIG. 7 is a timing chart of signals communicated between a camera and an interchangeable lens in the synchronous serial communication. PTL 1 discloses the following technology. Specifically, the synchronous serial communication is first performed at a first communication speed at which old-type interchangeable lenses support. If it is determined from the content of the communication that the interchangeable lens that is attached is a new-type lens, the communication speed of the synchronous serial communication is switched to a second communication speed that is higher than the first communication speed. Switching of the circuit of a communication terminal from an open drain type to a complementary metal oxide semiconductor (CMOS) type appropriate for the high-speed communication in the switching of the communication speed is also described in PTL 1. With this technology, a combination with an old-type interchangeable lens allows the operation although the communication speed is low and a combination with a new-type interchangeable lens allows the communication speed to be increased to a certain level.